Ab initio calculations on the isomerization of the benzene radical cation to the Dewar benzene structure and on possible pathways for the formation of C 4H 4 fragment ions from the benzene radical cation

Abstract

Ab initio calculations of possible fragmentation pathways for the formation of C 4H 4 fragment ions from the benzene radical cation show that the dissociation of lowest energy leads to the methylene cyclopropene radical cation. The calculated barriers for formation of the other classical C 4H 4 ion structures, vinyl acetylene, cyclobutadiene, and butatriene, are about 10 kcal mol −1 higher and almost identical. The fact that, in fragmentations of C 6H 6 precursors, besides the methylene cycloprene ion, only the vinyl acetylene ion is observed, should be ascribed to the fact that formation of this ion structure is much more direct than a fragmentation to the cyclobutadiene or the butatriene ion structure.